Vehicle cooling systems may include various cooling components such as heat exchangers, radiators, cooling fans and blowers, condensers, liquid coolant, etc. Additionally, the cooling system may receive cooling intake air from a front end of the vehicle, for example, through a vehicle or bumper opening, to assist in cooling the engine, transmission, and other components of the under-hood region. Such front-end air flow may add aerodynamic drag when the vehicle is in motion.
Various approaches may be used to reduce vehicular aerodynamic drag. One example approach is illustrated by Harich et al. in US 2008/0257286A1. Herein, the opening of one or more shutters and pivotable flaps in the frame of a vehicle cooling system may be adjusted, based on engine operating conditions, to thereby alter a total front end air-mass flow. Specifically, by adjusting the flaps and shutters of a variable geometry intake frame based on engine cooling demands, a cooling air flow through a blower and/or a coolant cooler may be adjusted. By restricting the shutters and/or flaps, for example, during conditions of low external air temperatures, air-flow and related aerodynamic drag may be reduced.
However, the inventors herein have recognized potential issues with such an approach. In one example, when the vehicle is moving, despite shutter and flap adjustments, some air may enter through the front end of the vehicle and rotate the blades of an unpowered fan of the cooling system. The free-wheeling fan may thereby lower cooling system resistance relative to a stationary fan which is not free-wheeling. The increase in cooling airflow thus generated when no cooling airflow is otherwise desired may lead to an increased cooling drag. As such, this may augment vehicular aerodynamic drag, thereby reducing vehicle performance and fuel economy.
Thus, in one example, some of the above issues may be addressed by a method of controlling a cooling fan of a vehicle cooling system comprising, during a first vehicle moving condition, operating the cooling fan, and during a second vehicle moving condition, selectively applying a braking torque on the fan. The first vehicle moving condition may include a request for airflow assistance from the cooling fan. The second vehicle moving condition may include no request for airflow assistance. Such an approach may be used with or without shutters or flaps, for example.
In one example, during a first vehicle moving condition, airflow assistance may be requested, for example, due to one or more under-hood components being heated above a threshold temperature. For example, an engine coolant temperature may be above a threshold, and airflow assistance may be requested to assist the radiator in cooling the coolant. Accordingly, a cooling fan of the vehicle's cooling system may be driven using power from the engine. During a second vehicle moving condition, airflow assistance may not be requested. Herein, the flow of air through under-hood components, due to the vehicle's motion, may provide sufficient airflow such that additional airflow assistance from a cooling fan is not required. As such, the fan may be “free-wheeling”, that is, the fan may be rotating due to the flow of ram air through the fan blades, and may not be driven by the engine. During such a free-wheeling condition, an engine controller may selectively apply a braking torque on the rotating fan blades to stop or reduce fan rotation.
For example, the braking torque may be selectively applied during a condition where the vehicle speed is above a minimum speed and the coolant temperature is below a lower threshold. Herein, the lower coolant temperature may not necessitate airflow assistance from the cooling fan. However, the rotation of the free-wheeling fan, while the vehicle is moving at the higher speed, may increase drag on the moving vehicle and reduce fuel economy. The drag may also reduce vehicle performance. Thus, during such a condition, a braking torque may be applied on the fan to reduce fan rotation (for example, reduce to a minimum speed or reduce to a halt). By selectively stopping or reducing fan rotation, air flow through the fan may be reduced when possible, thereby reducing the related aerodynamic drag. In one example, applying a braking torque may include shorting the power feed of an electric fan's motor so that the back EMF generated by the motor attached to the free-wheeling fan provides the braking torque. In another example, the braking torque may be applied mechanically, for example, using a latch or pin. Further, the braking torque may be adjusted based on vehicle operating conditions.
In this way, by selectively reducing free-wheeling of a cooling fan in a moving vehicle, based on operating conditions, cooling airflow across the fan may be reduced under selected conditions, thereby reducing cooling drag when such airflow is not otherwise needed. By reducing cooling aerodynamic drag when possible, vehicle fuel economy may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.